1. Field of the Invention
The present invention is in the field of electronic circuits. The present invention further relates to analog integrated circuits. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into a larger integrated circuit.
The invention also falls within the field of voltage reference circuits and also in the field of solid-state thermal sensors. These devices have been common in many electronic systems. More specifically, the invention falls into the class of all the analog integrated circuits that make use of the forward base-emitter voltage of a bipolar transistor.
2. Brief Description of Related Art
Integrated circuits commonly make use of accurate voltage references and thermal sensing elements that are based on the forward base-emitter voltage of an npn or pnp transistor. The most typical and known circuit is the bandgap voltage reference in its many topologies. Generally, it is desired that the reference voltage generated by the bandgap circuit be substantially invariant, both with temperature and with variations in process parameters.
The theory behind the function of the bandgap circuit is well known and was first introduced by Widlar (“New Developments in IC Voltage Regulators”, R. Widlar, IEEE Journal of Solid State Circuit, vol. 6, pp. 2–7, February 1971 and U.S. Pat. No. 3,617,859) and then described in several publications including “A Simple Three-Terminal IC Bandgap Reference”, A. Paul Brokaw, IEEE Journal of Solid State Circuits, vol. SC-9, No. 6, December 1974.
The main concept is to balance a PTAT (proportional to absolute temperature) term, generally generated by a Delta Vbe (ΔVbe), with a term that has a negative temperature coefficient, commonly derived from the Vbe (forward base-emitter voltage) of an npn or pnp. The resulting voltage reference is typically about 1.2V, silicon bandgap voltage. Similarly thermal sensors and solid-state thermostats make use of the known, predictable and relatively linear negative coefficient of the Vbe to generate voltages function of the temperature and/or to activate circuits only at pre-determined temperatures.
Conventional bandgap based voltage references and solid state thermostats do not offer a great accuracy, because the Vbe term is subject to variations with standard wafer manufacturing process parameters variability. It is widely recognized that many parameters contribute to the variability of the conventional bandgap voltage references, with the Vbe being by far the most important. Generally the PTAT term is much better controlled being mainly subject to matching errors (either currents, resistors or emitter areas matching).
The most widely utilized technique to achieve accuracy in integrated voltage references is to trim the output voltage by modifying the values of resistors in the circuit, in order to increase or decrease the PTAT term with respect to the Vbe term. The most conventional trimming techniques are either zener-zapping, metal fuses, poly fuses and EEPROM non-volatile memory. After the circuit is fabricated, the voltage reference is measured and the trimming operation is performed to adjust the reference.
As shown in FIG. 1, a typical trimming technique applied to a bandgap circuit 1, in the known Brokaw configuration, is aimed at modifying the value of the resistors R1 or R2 after the integrated circuit is fabricated. Although it is widely recognized that the main source of error in the voltage reference circuits is the Vbe variation of the transistor Q1, typically the Vbe is not directly corrected by trimming. Instead the PTAT term is corrected by trimming the values of the resistors R1 and R2 to compensate for the Vbe error.
However the trimming techniques are not desirable because they require large silicon area and longer test time to actually perform the trimming, contributing to the manufacturing costs of the device.
Accordingly, what is needed is a circuit to correct for the Vbe variations and to generate a process insensitive npn or pnp forward base-emitter voltage. This would allow the implementation of more accurate analog integrated circuits and in particular more accurate bandgap voltage references and thermal sensors without the need of very complex and expensive trimming techniques or with a reduced set of trimming elements.